Shimano doubles down...
"There has been a lot of speculation since Shimano first announced a new brake mount specific to road, but little in the way of details. Ironically coming on April 1st in Japan, Shimano is finally showing their hand and introducing two new hydraulic road disc brakes built specifically for the Flat Mount fitment. Announced along with a new 10 speed Tiagra group and WH-RX010 road disc wheelset, the new BR-RS505 and BR-RS805 disc brakes are non-series components at the 105 and Ultegra level respectively.
We have a feeling we’ll be learning more about these new brakes come Sea Otter, but the preliminary details are after the break…"
Beneficial to both the frame and the caliper design, Flat Mount uses an all new caliper that bolts up from the bottom instead of through the top like most calipers. On the rear brake that results in a true “Flat Mount” with the caliper sitting directly on the chainstay. Caliper mounting bolts are inserted through the bottom of the chainstay and thread into the caliper making a very tidy rear triangle of the frame. While the mounting and shape of the caliper changes, the brakes still use one-way bleeding with a bleed nipple at the highest point in the caliper’s hydraulic workings.
Due to the fact that the caliper bolts up through the bottom, the front brake will still need an adapter which is then bolted to the back side of the fork leg. Shimano tells us that you can still use additional adapters to run larger rotors while the brakes are capable of running 140mm.
Visually, there is little difference between the BR-RS505 (105 level) and BR-RS805 (Utlegra Level), other than the finish. We wouldn’t be surprised to find that there is more to it beneath the surface though.
Both calipers will use new brake pads which are not mountain bike compatible, likely meant for much higher heat applications.
Completing the RS-505 level hydraulic system is the new ST-RS505 hydraulic/mechanical shift lever which brings Shimano’s hydraulic road performance down to the most affordable level yet. Compatible with either of the new Flat Mount disc calipers or even previous Shimano hydraulic road calipers, the 11 speed mechanical shift lever with Vivid Indexing includes reach and free stroke adjustments as well.
As promised, frames equipped with the new Flat Mount standard will still be able to run standard road brake calipers with one of the Shimano adapters. That means frames will be backwards compatible, but the new Flat Mount brakes will not be since they can only be mounted to frames with the Flat Mount provisions.
Along with the ST-RS505 brake levers, the RS505 system includes a new 105 level Centerlock hubset to facilitate custom wheel builds that are 11 speed road disc compatible.
The Flat Mount brakes will continue to use the current Shimano Centerlock rotors with the SM-RT81 and SM-RT99 Freeza rotors available down to 140mm.
We’ll update the story as pricing and weights become available."